In recent years, it is proposed that a silver paste containing fine silver particles is used as a bonding material to be arranged between articles to be heated for a predetermined period of time while applying pressure between the articles, to sinter silver in the bonding material to bond the articles to each other (see, e.g., Japanese Patent Laid-Open No. 2011-80147).
When such a bonding material is used for fixing an electronic part, such as an Si chip, on a metal substrate, such as a copper substrate, after a silver paste containing fine silver particles dispersed in a solvent is applied on the substrate, the silver paste is heated to remove the solvent to form a predried film on the substrate to arrange the electronic part thereon, and then, the predried film is heated while applying pressure on the electronic part, so that it is possible to bond the electronic part to the substrate via a silver bonding layer.
However, in the method disclosed in Japanese Patent Laid-Open No. 2011-80147, the leveling of the surface of the predried film is not always good due to the dispersion failure, print failure and so forth of the silver paste. Therefore, in order to satisfactorily bond the electronic part to the substrate, it is required to cause the surface of the predried film to be flat by increasing pressure applied to the electronic part. For that reason, when a large and thin electronic part, such as a recent Si chip, is bonded to a substrate, there is some possibility that the surface of the electronic part may be warped or broken by increasing a load applied during the bonding of the electronic part to the substrate.
In order to eliminate such problems, there is proposed a method for forming a flat coating film by adding a viscosity modifier of a structure having an ether bond, such as 2-butoxyethoxyacetic acid or 2-methoxyethoxyacetic acid, to a silver paste containing fine silver particles (see, e.g., WO2012/169076).
However, in the method disclosed in WO2012/169076, the viscosity modifier of 2-butoxyethoxyacetic acid, 2-methoxyethoxyacetic acid or the like is added to the silver paste containing fine silver particles as a dispersant to form the flat coating film. For that reason, when an electronic part is bonded to a copper substrate by the silver paste, the dispersant reacts with the surface of the copper substrate to produce mottled protrusions of aggregates on the surface of the copper substrate to form cracks in the predried film, so that the cracked portions exist as cavities (voids) to easily deteriorate the bonding strength of the electronic part to the copper substrate. In particular, it is desired to increase the thickness of the predried film in order to obtain the stress relaxation thereof. However, if the thickness of the predried film is increased, the solvent in the predried film is evaporated at a stroke during the formation of the predried film to shrink the predried film, so that cracks are easily formed in the predried film.